This invention is a circuit for maintaining at a constant level the photomultiplier output in a raster input scanner (RIS) or the light level of the scanning beam in a raster out scanner (ROS), and more specifically is a circuit for determining the amount of deviation of the light level from the desired level during a calibrate phase, and using that correction value during the actual reading or writing phase of operation for normalizing the output.
In a raster input scanner of the type having a rotating polygon, a beam of light illuminates one polygon facet at a time to produce a flying spot. This in turn illuminates one line of the original, and the reflected light is received by a photomultiplier to form one line of video in the form of an analog voltage.
A problem in polygon systems is that the light intensity varies across the line as a function of spot speed which may be slower at the center of the line, as a function of energy distribution along the line due to changing optical parameters (such as efficiency), and as a function of the angle between the original and the beam, which is off normal at the ends of the line. These and other factors affect the light intensity, the corrections for which may add cost or other complications to the system.
One prior correction system uses a preliminary calibrate procedure comprising the scanning of a test strip. The amount of light is measured by the photo multiplier tube (PMT), the analog output voltage of which is compared to a threshold. A correction voltage is determined from this comparison, and is stored. Later, during the regular operation of the system, this correction voltage is added back into the PMT output to maintain at the output of the circuit a constant voltage for a constant level of original brightness. However, there are problems with this system. Correcting the gain of the amplifier circuit changes the bandwidth of the system, and there is no assurance that the correction is accurate. These problems become more important in a color system where variations of bandwidth and light levels may distort the colors.
Raster output scanners are similar. An intensity modulated flying spot illuminates a xerographic drum, or the like, and variations in the basic intensity of the light at the drum may create color distortions.
What is required is a system for correction of light variations that will not change the bandwidth of the amplifier circuits, and one that will make accurate corrections.